This invention relates to a new process for the preparation of 3-hydroxycephalosporins. These compounds are disclosed and claimed in U.S. Pat. No. 3,917,587. The process disclosed in the copending application for preparing 3-hydroxycephalosporins involves treatment of a 3-exomethylenecepham compound with ozone to form an intermediate ozonide followed by decomposition of the ozonide in the presence of a reducing compound to produce the desired 3-hydroxycephalosporin.
The novel process for preparing 3-hydroxycephalosporins by this invention comprises ring-closure of 3-hydroxy-4-bromo-2-(2-formylthio-4-oxo-3-amido-1-azetidinyl)-2-butenoate esters or their corresponding 3-imido compounds. These compounds are alternatively named as azetidinones and, as such, can also be called 1-(1-protected carboxy-2-hydroxy-3-bromo-1-propenyl)-3-amido(or imido)-4-formylthioazetidin-2-ones.
The ring-closure reaction described herein is accomplished by treating the above butenoate esters with 1,5-diazabicyclo[3.4.0]non-5-ene (DBN) or 1,5-diazabicyclo[5.4.0]undec-5-ene (DBU).